In an unloaded transmission line which is not compensated by inductors, DC voltage remains at the transmission line after a circuit breaker breaks the transmission line. It is known that if the circuit breaker is reclosed in the state that the DC voltage remains, an overvoltage (closing surge) is generated. The magnitude of the overvoltage becomes several times the voltage of the system. If the large overvoltage is generated like this, it is worried that the insulation of the equipments installed in the system is affected.
For this reason, as a method to suppress the overvoltage at the time of closing the unloaded transmission line, it is known to provide a circuit breaker with a making resistor. In the 500 kV system in Japan, for example, a circuit breaker with a making resistor is employed so as to suppress the overvoltage like this. In the circuit breaker with a making resistor, a making resistor contact composed by connecting a making resistor and a contact in series is connected in parallel with a main contact of the circuit breaker. In the circuit breaker with a making resistor, the contact of the making resistor is closed prior to the main contact of the circuit breaker. By this, the overvoltage is suppressed.
On the other hand, in the unloaded transmission line which is compensated by an inductor, an oscillation voltage due to its electrostatic capacitance and inductance is generated in the transmission line after the circuit breaker breaks the transmission line. In such a case, if the circuit breaker is reclosed at the time point when the voltage between contacts of the circuit breaker is large, an overvoltage is generated. As an overvoltage suppressing method at the time of reclosing the transmission line compensated by the inductor, a method to control the closing phase of the circuit breaker is known. This is a method in which the circuit breaker is reclosed at the time point when the voltage between contacts is small. As a method to estimate the time point when the voltage between electrode contacts becomes small, the following is known.
As one method, a method to close the circuit breaker at the optimum timing by function approximating the voltage between contacts of the circuit breaker is disclosed as indicated in the following. Firstly, the power source voltage is assumed as a sine wave of the commercial frequency. In addition, if the oscillation voltage at the line side is composed of a single frequency, it can be assumed as a sine wave, too. The voltage between contacts is estimated by a function approximating both the voltages by the sine wave. The closing time of the circuit breaker is decided based on the voltage between contacts.
As a second method, a method to close the circuit breaker at the future zero point of the voltage between contacts by measuring the time between the zero points of the voltages between contacts of the circuit breaker is disclosed as indicated in the following. A time between the zero points of the voltage in a cycle of the voltage between contacts after breaking, and a time between the zero points of the voltage in a next cycle of the voltage between contacts is measured. If these two times between the zero points of the voltage between contacts are the same, the frequency of the voltage between contacts can be found. By this, the future zero point of the voltage between contacts can be estimated regardless of the waveform of the voltage between contacts.
However, in the above-described overvoltage suppressing method, there are following problems, respectively.
In the case that the overvoltage is suppressed using the circuit breaker with resistive element, it is necessary to add specifically the circuit breaker with resistive element to the usual circuit breaker. For this reason, in the case that the circuit breaker is seen as a whole, the circuit breaker may become large in size.
In the transmission line, there is a case that an inductor is installed so as to compensate its capacitive power. When the circuit breaker opens the transmission line where the inductor is installed, the voltage oscillation of the frequency determined by the electrostatic capacitance of the transmission line and the inductance of the inductor is generated in the transmission line. The frequency of the voltage oscillation of the line is generally different from the frequency of the power source voltage. In this case, the voltage between contacts of the circuit breaker becomes in the aspect of the multi frequency wave.
In this time, in the case that the circuit breaker is closed at the optimum timing by a function approximating the voltage between contacts of the circuit breaker, there is a following problem.
In the electrostatic capacitance of the transmission line to determine the frequency of the voltage oscillation of the line, there is a component to ground of the self-phase, a mutual component between the other phases, and a component to ground of the other phases. These electrostatic capacitances become different values for each phase depending on the geometric layout of the transmission line. For this reason, it is very rare that the oscillation waveform of the line side voltage becomes a sine wave with a single frequency. Generally, in many cases, the oscillation waveform itself is already in a multi frequency wave state. In this case, it is difficult that the voltage oscillation of the line in itself is made function approximated. Accordingly, it is actually extremely difficult to obtain the voltage between contacts from the function approximation.
In the case that the circuit breaker is closed in the state that the voltage is applied between contacts of the circuit breaker, when the voltage between contacts becomes larger than the dielectric strength between the contacts, discharge is generated between the contacts. If the discharge is generated, the circuit breaker is in electrical contact before the contacts they are in mechanical contact. This discharge is called as the pre-arcing.
Here, in the case that the voltage between contacts of the circuit breaker is in the multi frequency wave state, there may be a case that a wave height value of the voltage becomes not less than the power source voltage. In this case, if it is tried to close the circuit breaker when the voltage between contacts is at the zero point, it may be possible that the circuit breaker becomes in the closed state by the discharge due to the above-described pre-arcing at the time point when the voltage between contacts is large. In this case, a large overvoltage is generated. Thus, in the case that the voltage between contacts is in the form of a multi frequency wave, it is not possible to suppress the overvoltage by measuring only the zero point of the voltage between contacts.
An object of the present disclosure is to provide an overvoltage suppressing device which can suppress an overvoltage generated at the time of closing a circuit breaker even in the case that the voltage between contacts of the circuit breaker is in a multi frequency wave.